Planar phased arrays of RF radiators wherein each radiator is capable of transmitting and receiving RF electromagnetic signals are generally well known. Such controllable radiators ideally have minimum size, weight, cost and complexity along with low insertion loss, temperature stability, phase accuracy, low drive power requirements, etc.
Proper spacing and control of the relative phasing and amplitude or attenuation of the radiator signals allows the production of a precisely defined overall radiation pattern whereby a well defined beam may be electronically pointed and shaped by way of digital control signals supplied by way of an array controller/computer.
Typical uses of such phased arrays may be found in airborne or land-based radar systems operable in the upper frequency ranges, such as the S, C, X- and K-bands. Such uses require high performance RF transmitter/receiver (T/R) or radiator modules wherein the aforementioned insertion loss, phase accuracy and switching time, as well as bandwidth, switching power and, of course, costs are critical.
As indicated in copending U.S. patent application Ser. No. 07/795,026 referenced above, conventional active T/R modules for use in phased arrays may be of many different types. FIG. 1 of the present application schematically illustrates a typical such module with separate transmit/receive feed ports 1 and 2, respectively. The transmit leg may typically include a phase shifter 3 and controllable attenuator 4 along with a relatively high power amplifier 5 connected to a standard microstrip circulator 6 by way of line 7 for communicating RF signals to antenna 8. As to the receive leg, microstrip line 9 communicates RF signals to transmit/receive limiter 10, low noise amplifier 11, controllable attenuator 12 and a phaser 13, which in turn is connected to the output port 2.
Appropriate phase and amplitude settings of the radiators of the phased array wherein there are typically 2000 such elements may be determined by an array controller/computer (not illustrated) for setting the phase shifters, for example, at desired relative phase shifts for both transmitting and receiving purposes. Moreover, the appropriate radiation pattern shape, pointing angle, etc., may be obtained without any mechanical movement of the array or movement of the radiator elements.
Although improvements continue to be obtained in the art, still further improvements are required for many applications. For example, problems continue to exist in T/R modules, such as power consumption, heat removal, receiver protection, receiver dynamic range and, of course, cost.
Of particular note is the limited dynamic range of existing transmit/receive modules which present a problem due to the third order intermodulation products produced by the low noise amplifier (LNA). Such third order intermodulation productions are considered to be the most troublesome since they fall within the bandpass of even moderate bandwidth amplifiers. That is to say, a typical T/R module circuit, as illustrated in FIG. 1, may have a respectable receiver dynamic range of 100 dB when operating in a spurious free environment. For the spurious free case the dynamic range of a receiver is usually defined as the range from the 1 dB gain compression point of the LNA to the system noise level. However, when operating in spurious environment, such as where aircraft are flying in formation or in a vicinity with radars operating in the same frequency band, the dynamic range of the receiver may be significantly reduced due to the noted third order intermodulation products produced by the low noise amplifier. The dynamic range in a spurious environment is usually defined as the range from the 1 dB gain compression point to the interfering spurious level of the receiver.
We have discovered a unique technique for obtaining a digital latching bandpass filter utilizing a resonant ring circuit which is low loss, fast switching and microstrip compatible. We have further discovered that such electronically tunable bandpass filters may be used in conjunction with the low noise amplifier (LNA) to increase the dynamic range of the receiver in active T/R modules for phased arrays.
In this regard our tunable bandpass filter may be implemented using a resonant ring structure including two couplers, an LNA and a phase shifter wherein the phase shifter may have up to 360.degree. of phase shift, and the resonant frequency of the ring can be tuned anywhere in the operating frequency range. In many applications less than 360.degree. of phase shift will be required because of the limited tuning bandwidth necessary. The ring structure disclosed in more detail below may be used to replace the LNA of FIG. 1. Moreover, the resonant ring filter functions so that the input signal coupled to the ring and the signal progressing around the ring constructively interfere at the input of the LNA when the ring is resonant. The second coupler is used to couple the output signal which has the bandpass characteristic of the ring, and a rejection of off-frequency spurious signals will occur due to the passband response of the filter. In this manner, the third order intermodulation products will be reduced by a factor of 3 times the filter rejection.
Additionally, if desired, our resonant ring structure may be used as a band rejection filter if the output port is selected to be the port loaded on the input coupler. In this regard, at resonance and with proper parameter selection the power would be reinforced in the ring and the power at the output port reduced.
Still further, although the herein disclosed tunable filter can easily obtain a single passband response over desired bands, multiple passband responses may occur depending upon the electrical length of the loop and the required operating bandwidth. We have discovered that if larger operating bandwidths are necessary and the electrical length or insertion phase of the ring structure cannot be reduced to eliminate the multiple responses, such responses may be eliminated through the use of an additional stage in which the electrical length of the loop is different than the first.
Receiver protection advantages may be also be obtained by our tunable filter since the T/R limiter and LNA are placed in the ring structure and are decoupled from the antenna. Thus, off resonance signals are terminated in a load. Additionally, reflection from the T/R limiter back to the antenna is significantly reduced and, therefore, reflection uniformity for low RCS (radar cross section) applications is considerably improved. The limiter and LNA are also less susceptible to burnout from spurious signals. Furthermore, if desired, the ring can be de-tuned either with the phase shifter or LNA to thus offer still further protection of the T/R limiter and LNA against the high powered amplifier during the transmit pulse.
As a still further application of our tunable filter, if desired, the receiver output port may be switchably connected to a path providing a filtered response from the active filter when in a spurious environment or connected to a path excluding the active filter when operating in a broad band, spurious free environment. As a still further variation of the latter alternative, if the filter remains energized and tuned to a single tone spurious signal, a band rejection within the broad band would exist.